The Best Ways to Build in Blockwork

In the excitement over new ways of building that we often seem to read about, it’s easy to overlook the fact that one form of construction dominates the housebuilding market in the UK, both for professionals and self builders.

It’s known variously as traditional construction, masonry build or brick and block.

Masonry Facts

Concrete masonry walls have been the dominant form of wall construction in the UK since the 1930s

Issues today involve the width of the cavity itself, how much it is filled with insulation and how it is built to avoid penetrating water across wall ties and above windows and doors

Innovations such as Porotherm, thin-joint and aircrete are slowly transforming the way we build block walls

A Brief History

Its historical roots lie in the use of brick to build external walls which goes back to the rebuilding of much of London after the Great Fire in 1666. Before that, most homes had been built using timber.

Solid wall brick construction became the backbone of building in the Georgian and Victorian eras, although stone was often used instead in areas where there were good local quarries. The cavity wall, one of the key features of contemporary masonry construction, didn’t appear until late Victorian times and didn’t become commonplace until the 1920s.

So why the change? The short answer is that the arrival of the mass-produced concrete block made it cheaper and quicker to split the external wall into an inner and outer skin so that the amount of relatively expensive brick or stonework required was halved. It also addressed the issue of damp — one of the main failings of the solid brick wall, because the cavity provides a line of defence against all manner of water penetration.

In a cavity wall, the load-bearing is taken care of by the utilitarian concrete blocks used on the inner skin, while the outer skin brick or stonework just has to keep the weather at bay. You can tell very easily whether a brick wall is solid or cavity work by the pattern of the bricks used. Stretcher bond, where you see only the long face of the brick, was almost unknown before the 1920s but is now almost ubiquitous.

Cavity Issues

The cavity wall may have become the predominant form of construction in the UK but the cavity itself is not universally loved. It’s very simple for architects to draw and detail a cavity wall design, but it’s much harder to build well and, as the cavity itself gets closed in, there is ample room for mistakes to be made during the construction phase.

One of the elements that causes problems is the placing of the wall ties, required to stitch the inner and outer skins together structurally. These ties can get bent out of shape, they can rust and they can also collect mortar droppings, known delightfully in the trade as ‘snots’, which act as a pathway for rainwater to cross the cavity and get into the blockwork.

There has always been an assumption that water will find its way across the cavity and that there may be residual water trapped inside the cavity. Consequently, whenever the cavity is breached, typically where windows or doors are fitted, the opening above has to be made watertight so that any water trapped in the cavity is directed outside.

Most builders deal with this issue by using steel lintels with in-built cavity trays, but there are details such as roof abutments where more intricate detailing is required. In fact, a substantial industry providing solutions to the problems of water ingress into cavities (cavity trays and weep holes) has developed.

Insulation

The cavity story gets even more complicated by the more recent innovation (well, this one is a mere 30 years old) of placing insulation inside it to beef up the thermal performance of masonry-built homes. Whereas the alternative building systems invariably set out to provide excellent insulation levels, cavity wall work has struggled to come to terms with the conflicting aims of keeping rainwater out and heat in.

Yet over time, both the designs and the standards have improved and there are now well-worked routines for how you should build cavity walls. The Building Regulations split the nation into four climate zones, each with its own risk profile and each with its own prescriptions of how insulated cavity walls should be built (below). And paradoxically, the move to pack more insulation into cavities has resulted in much wider cavities which seem to be less prone to water penetration.

This map highlighting ‘UK zones for exposure to driving rain’ is used to determine suitability for full-fill cavity wall insulation. As you can see, west-facing walls in western areas are most prone

However, this has contributed to another issue for masonry builders — the width of the wall. On larger homes, this is not critical, but on smaller houses the area taken up by external walls can have a significant effect on room sizes. Framed construction systems in particular can be used in conjunction with lightweight rainscreens such as timber or tile and achieve the required U value in less than 300mm width where conventional masonry cavity work is struggling to do this in 400mm.

The Advantages

Despite all these many issues, cavity wall work remains by far the most common method throughout the UK, with the exception of Scotland which embraces timber frame. Although cavity-based masonry work can be relatively slow, is liable to weather disruption and can be prone to error, it is also the best understood system and the most readily available.

Every builder is familiar with the cavity wall and every builders’ merchant stocks all the materials required to build one.

It’s also a forgiving system: if the foundations are not quite as true as they should be, good bricklayers can sort out any issues within their first few courses.

Blockwork is equally at home with extensions as it is with new build — another area which factory-built systems can struggle to cope with.

Innovations

Thin-Joint
Despite many people predicting the “end of the cavity”, it’s actually proven to be a remarkably resilient form of construction. However, there are innovations happening in the world of masonry builds which involve speeding up the block-laying process and narrowing the wall profile. These are known collectively as thin-joint systems which, ironically, turn the clock back to the pre-cavity days when external walls were solid.

Thin-joint block laying does away with the conventional 10mm bed of cement mortar and replaces it with a 2-3mm glue mortar bed, similar in characteristics to tile adhesive. It sets very rapidly and thus enables blocklayers to use much larger blocks and to build them into walls much faster. The larger the project, the bigger the labour savings from employing thin-joint methods, and when used wisely, it brings with it many of the speed benefits of off-site construction methods.

The big manufacturers all offer thin-joint solutions in aircrete, the familiar material used by blocklayers throughout the UK. But there is also a move to use honeycomb clay blockwork. It’s extremely common on the Continent but has rarely been used in the UK till recently.

It shares some of the benefits of other thin-joint systems in that it is quick to lay and to get up to storey height in a day. But additionally it promises better insulation performance because of the honeycomb structure of the blocks. With the backing of Wienerberger, the world’s largest brick manufacturer, Porotherm clay blocks are slowly making inroads into the conservative block market, marketed not just on speed but also on sustainability grounds.

Thin-joint solutions are not new: Tarmac Durox’s thin-joint system has been available in the UK since the 1990s. While great things were predicted for thin-joint masonry, the switch to new forms of construction nosedived during the building slump following the 2008 financial crash and traditional blockwork came back into the ascendency.

Interest in all novel construction forms seems to rise and fall with the larger economic cycle and now that the building business is picking up again, we are seeing labour rates rise. Once again, masonry builders are looking at ways of reducing labour times spent on site and interest in thin-joint work is set to rise again. For the time being though, something like 95 per cent of masonry work is still conventional cavity work.

Weinerberger’s Porotherm clay block

Blocks may look similar now to the way they did years ago, but the choice is enormous — in particular between aircrete (with its better insulating properties and lighter working loads — it can also be cut easier) and denser blocks, used mainly now below ground and for additional load-bearing requirements. Wienerberger’s Porotherm clay block (top right) is a recent innovation, with an interlocking system meaning no mortar is required on the vertical joint

The Future
The two main drivers behind the quest for new walling methods are the need to reduce labour costs on site and the need to meet higher building standards, especially as regards thermal insulation levels. The traditional builders have responded to both challenges largely by slow adaptation rather than introducing wholesale changes and this possibly suits a British housebuilding industry that is more interested in building cheaply, rather than well or quickly.

Consequently, the cavity wall will be with us for some time yet and it seems quite capable of adapting to step-changes in insulation standards that lie ahead. Small changes such as switching to better wall ties or more refined detailing around window and door openings may end up being more significant than entirely new methods. And perhaps this suits a construction industry that is based around casual work gangs with a limited skill base.

Whereas countries like Japan are able to bring forward off-site construction with houses built by robots, we in Britain are deeply wedded to craft-based building and the cavity wall is therefore something to be cherished — not discarded in the name of progress. For while it seems in many ways an illogical and outmoded way of building homes, it is also a routine which almost all of our established builders are familiar with and needs very little in the way of specialised knowledge.